The use of various grinding or pulverizing machines for reducing the particle size of various solid materials, such as rocks, ores and minerals, is well known. The use of screens or nests of screens to separate particles into various ranges of sizes is also well known. Very often the screens are vibrated to effect better, faster separations.
The separation of rotund beads from irregular shaped particles on a slanted surface is taught, e.g., in French Pat. No. 730,215; U.S. Pat. Nos. 1,976,974; 2,778,498; 2,658,616; and 3,464,550. A U.S. Department of Interior, Bureau of Mines publication R.I. 4286 dated May, 1948 on "New Dry Concentrating Equipment" contains information on a vibrating-deck mineral shape separator; the separator disclosed is a vibrated tilted table where trajectory of particles across the surface is dependent on the shape of the particles. There are various sludges and slags from mining and metallurgical operations which are known to contain inclusions of metal droplets, such as copper, nickel, tin, and others.
U.S. Pat. No. 3,037,711 teaches the use of beater mills or hammer mills for pulverizing dross from metal particles, then separating the fines from the particles by suction.
General information about pulverizers, screens, and tabling may be found in, e.g., "Chemical Engineers Handbook" by Robt. H. Perry, Editor, published by McGraw-Hill.
The separation of solid Mg metal spheroids from entrapment in a solid contiguous matrix of a friable salt or mixture of salts presents particular problems to an investigator who may desire to recover the Mg in its spheroidal form and also retain on each spheroid a thin protective coating of the matrix material. Whereas it has been known for many years that such a Mg-containing matrix is removed as cell sludge from the electrolysis of molten MgCl.sub.2 and as a slag material from Mg or Mg-alloy casting operations, attempts to recover the Mg or Mg alloy particles by grinding or intensive ball-milling have generally resulted in smashing, breaking, or flattening a large portion of the Mg particles. Such deformed particles may be acceptable if the primary purpose of recovering the Mg metal is that of re-melting the Mg for coalescence or for re-casting.
In the present invention, however, what is of special interest is the recovery, from the solid matrix, of Mg spheroids which each have a thin protective coating of the matrix remaining thereon. Such spheroidal Mg particles are of particular interest for use in inoculating molten ferrous metals, e.g., the desulfurization of steel. The thin protective coating of matrix on the Mg spheroids helps avoid the hydrolysis of Mg by moisture or the oxidation of Mg by air. Mg particles which are substantially flattened or elongated or which do not have a high degree of rotundity are not as readily useful in operations where the particles are injected through a lance beneath the surface of molten iron or steel. Ideally, the operators of such lances would prefer that the Mg particles be of consistent size, consistent Mg content, and consistent rotundity in order to avoid unwelcome variances during the inoculation process.
U.S. Pat. No. 3,881,913 and U.S. Pat. No. 3,969,104 disclose the preparation of salt-coated Mg granules by an atomization technique and also disclose that such granules are useful for injection into molten iron through a lance.
The salt-coated Mg particles of interest in the present invention may be called "powders", "beads", "pellets", "granules" or other such term. The particles of greatest interest have a high degree of rotundity, being of a spherical and/or oval shape, and have a particle size in the range of about 8 mesh to about 100 mesh (U.S. Standard Sieve size). For the common practice of inoculating ferrous melts through a lance, the preferred particle size is generally within the range of about 10 mesh to about 65 mesh.
Various patents have described the molten salt mixtures, containing MgCl.sub.2, which may be employed in electrolytic cells for the electrolytic production of Mg metal, e.g., U.S. Pat. Nos. 2,888,389, 2,950,236, and 3,565,917. It is disclosed that the composition of the salt mixture may be varied in order to adjust the density to be greater than, or less than, molten Mg metal. Sludges formed in such electrolytic Mg processes are known to contain Mg metal particles entrapped in a matrix of salt, and, usually there are some Mg oxide values also present, due to contact with air or moisture. The use of fluorides in the salt mixtures as coalescing agents for the Mg metal is disclosed.
At the 6th SDCE International die casting congress, organized by The Society of Die Casting Engineers, Inc. at Cleveland, Ohio on Nov. 16-19, 1970 there was a paper (Paper No. 101) on "Factors Controlling Melt Loss in Magnesium Die Casting", authored by J. N. Reding and S. C. Erickson. The paper discloses the entrapment of Mg particles and Mg alloy particles in sludges and slags, and discloses studies about coalescing agents and dispersion agents (emulsifiers) for the Mg particles. It also discloses the grinding, in a ball mill, of a Mg-containing sludge to recover the Mg particles from entrapment therein.
Therefore, sludge material from Mg-producing processes, or from Mg-casting operations are known to contain Mg metal entrapped therein. In the Mg-producing processes, e.g., the electrolyzing of molten MgCl.sub.2 in the presence of other molten salts to produce Cl.sub.2 and molten Mg, the sludge material is composed of metal salts, oxides, impurities, and contaminants and contains a relatively small amount of Mg particles of various sizes dispersed therein.
In Mg-casting, or Mg-alloy casting, a melt flux is usually provided on the surface of the molten metal in the melting vessel to prevent or retard contact of the metal with air or moisture and to prevent Mg fires. Such fluxes are usually mixtures of molten salts such as disclosed in U.S. Pat. No. 2,327,153 which also discloses that small Mg beads become trapped in the frozen sludge or slag as discrete fine globules having a diameter as small as 0.01 inch. The patent also discloses the invention of re-melting and stirring the sludge or slag in order to get the small Mg beads to coalesce into large beads of about 0.5 inch or large diameter, then partly cooling and separating the frozen beads from the still-molten salts by filtration.
Thus, the metal salt compositions of Mg cell sludges, Mg-casting slags, and Mg alloy-casting slags are a matter of record and are seen to comprise various mixtures and ratios of alkaline metals salts, alkaline earth metal salts, some oxides and, generally, some impurities and contaminants.
It is an object of the present invention to recover rotund metal particles, Mg particles, or Mg alloy particles, from entrapment in a contiguous, friable matrix of sludge or slag material.
A further object is to recover such rotund particles by a process which substantially avoids flattening, rupturing, or pulverizing said particles.
Another object is to recover such Mg beads in a manner that the Mg particles retain a thin protective coating of the sludge material in which they were entrapped.
Yet another object is to recover coated Mg particles having a relatively consistent Mg content and relatively consistent particle size range and rotundity for use as an inoculant through a lance into a molten ferrous metal.
These, and other beneficial objects apparent to practitioners of the relevant arts, are substantially attained by the presently disclosed invention.